The present invention generally relates to an image forming apparatus for forming an image by using a plurality of developing devices and more particularly, to control of density of toner in the image forming apparatus.
Generally, in known developing devices utilizing a developer composed of a mixture of toner and carrier, since the toner in the developer is consumed upon developing of an electrostatic latent image, it is necessary to perform replenishment of the toner corresponding, in amount, to a drop in density of the toner due to consumption of the toner for developing.
In order to control replenishment of the toner, there has been employed, for example, a method (referred to as "AIDC", hereinbelow) in which after a reference latent image is formed on a surface of a photosensitive (photoreceptor) drum acting as an electrostatic latent image support member and then, is developed into a reference image, density of the reference image is detected optically and the developing device is replenished with the toner in accordance with the detected value of the density of the reference image such that a content of the toner in the developer in the developing device is maintained at a constant value at all times.
Namely, the AIDC is based on such a principle that since the density of the developed image on the surface of the photosensitive drum is proportional to the content of the toner in the developer, changes in the density of the reference image are detected such that replenishment of the toner to the developing device is controlled in accordance with the changes.
Meanwhile, also in image forming apparatuses coming into wide use recently, especially in image forming apparatuses of a type in which a plurality of imaging units each constituted by a corona charger, an exposure means and a developing device are arranged around the photosensitive drum in a rotational direction of the photosensitive drum and toners having different tones are, respectively, contained in the developing devices such that multi-color images are formed, the density of the toner in the developer in each developing device is required to be maintained at a constant value by employing the AIDC.
However, in the known apparatuses of this kind, it has been so arranged that the reference images corresponding to the respective developing devices are formed on an identical line oriented in the rotational direction of the photosensitive drum and that developing areas of the respective developing devices are set at an identical location. Therefore, a reference latent image developed by a first developing device is afterwards developed repeatedly by second and third developing devices disposed downstream of the first developing device and thus, mixing of colors of the reference image takes place.
Needless to say, in order to obviate this problem, a compensating charger or the like is provided between the image forming processes and an electric potential of the reference image formed by the preceding developing device is dropped substantially to an electric potential of the background such that the toner is prevented from being recoated on the preceding reference image by the subsequent developing device. However, the compensated electric potential is not necessarily stable owing to such factors as environmental change with time, deterioration of the photosensitive material, variations in output of the compensating charger, etc. Hence, the known image forming apparatuses have such a drawback that mixing of the colors of the reference image is unavoidable. Especially, in the case where the first developing device contains color toner and the second developing device contains black toner, such a problem arises that when the black toner is recoated on the reference image formed of the color toner, quantity of reflected light varies excessively due to a large difference in reflectance between the color toner and the black toner even if amount of the black toner recoated on the reference image formed of the color toner is minute, thereby making it impossible to perform accurate control of replenishment of the toner.